The subject invention relates generally to systems for preventing removal of or relative motion between parts, to methods and apparatus for operating magnetic locking assemblies, to magnetic locks and similar fastening devices, and to various door structures including magnetic locks.
Magnetic locks have become known in which a keeper or striker plate is magnetically attracted to a pole face of an electromagnetic assembly. This type of electromagnetic lock generally works well when the striker plate and electromagnetic assembly are mounted on or connected to a door and doorframe, respectively, or to similar structure, in such a manner that a forced opening of the door or similar structure would urge the striker plate to separate itself against the force of the magnetic attraction in a direction parallel to the direction of the magnetic field or perpendicularly to the pole face of the electromagnetic lock.
In practice, this limits the utility of the discussed type of magnetic lock, since there are many situations in which the preferable or inevitable arrangement would have to be such that there occurs a shearing motion between the striker plate and the pole face or electromagnetic assembly of the lock, when attempts are made to open the locked door or other structure forcibly.
In cases of the latter type, existing electromagentic locks of the above mentioned kind have not been doing well, inasmuch as the retention force between the striker plate and electromagnetic assembly at the pole face is then determined practically by the friction between the latter two parts, rather than effectively by the strength of the magnetic field with which the electromagnetic assembly attracts the striker plate. In other words, the only significant force developed in the transverse direction parallel to the pole face is that of mechanical friction which is usually less than 25 percent of the magnetic attraction holding force perpendicular to the pole face.
In consequence, many swinging or lifting door installations and similar arrangements could not use the above mentioned type of lock safely. Also, mechanically actuated thrusting bolts frequently had to be employed in magnetic locks for a safe locking operation. This, of course, caused additional expense and complexity, and required that a fairly exact correspondence between the position of the thrusting bolt in one lock component and the bolt-receiving cavity in the other lock component be always maintained and consistently reproducible, even over a long period of time and even with door structures that may be subject to certain manufacturing and installation tolerances or to settling over prolonged periods of use.
Another drawback of bolt-type electromagnetic locks is that the bolt is subject to sticking upon deenergization of the lock, whereby people sometimes have been unable to leave a room controlled by such magnetic locks.